Ceramic composition having high adsorptive capacity for oxygen at elevated temperature

ABSTRACT

A ceramic composition having a high adsorptive capacity for oxygen at elevated temperature, including at least one of: Bi 2−y Er y O 3−d ; Bi 2−y Y y O 3−d ; La 1−y Ba y Co 1−x Ni x O 3−d ; La 1−y Sr y Co 1−x Ni x O 3−d ; La 1−y Ca y Co 1−x Ni x O 3−d ; La 1−y Ba y Co 1−x Fe x O 3−d ; La 1−y Sr y Co 1−x Fe x O 3−d ; and La 1−y Ca y Co 1−x Fe x O 3−d ; wherein x is from 0.2 to 0.8, y is from 0 to 1.0 and d=0.1 to 0.9. Such ceramic composition may be made using a modified Pechini synthetic procedure. The resulting ceramic composition is usefully employed as an adsorbent for separation of oxygen from an oxygen-containing feed gas mixture, e.g., in a pressure swing adsorption (PSA) process.

BACKGROUND OF THE INVENTION

1 Field of the Invention

This invention relates to a ceramic composition having high sorptivecapacity for oxygen at elevated temperature. Such ceramic compositionmay be employed for example in an elevated temperature pressure swingadsorption system for separation of oxygen-containing feed gas mixtures.

2. Description of the Related Art

In the field of gas separation for the production of industrial gases, avariety of technologies have been employed in the art, including:cryogenic separation processes involving cooling and pressurizing a feedgas mixture to form a liquid that then undergoes distillation;chemisorption or chemical reaction removal of unwanted gas species froma feed gas mixture to yield the desired gas component as the onlyremaining gas-phase product; scrubbing of the feed gas mixture to removeundesired soluble components therefrom, chromatographic separation ofthe feed gas mixture; and physical adsorption-based processes.

The latter approach of physical adsorption-based processes includespressure swing adsorption (PSA) in which a bed of physical adsorbentmaterial is contacted with a feed gas mixture including one or morecomponents for which the physical adsorbent material has sorptiveaffinity, to preferentially adsorb such components, while thenon-adsorbed components flow out of the contacting zone containing theadsorbent material. The adsorbent material then is lowered in pressurein relation to the pressure at which the feed gas mixture is contactedwith the adsorbent material, e.g., by a “blow-down” or depressurizationstep, or alternatively by vacuum desorption, whereby the previouslysorbed gas components desorb from the adsorbent material and aredischarged from the adsorbent material.

The foregoing PSA process may be carried out in a multiplicity ofadsorbent beds, joined together by valved manifolds at their respectiveinlet ends and at their outlet ends, and coupled at the inlet manifoldto a source of the feed gas mixture to be separated. In operation, thevalves are operated to carry out a cyclic, repetitive process in whichat least one of the beds is undergoing active processing of gas mixture,while another or others are off-line or undergoing regeneration. Thus, afirst bed of a multibed PSA system may be undergoing pressurization withfeed gas mixture, while a second bed undergoes depressurization anddischarge of previously sorbed gas therefrom. The regeneration mayentail use of a purge or displacement gas, or use of embedded heatexchange coils to aid in desorbing gas from the bed.

A wide variety of sorbent materials have been used or proposed for usein PSA systems, including zeolites, activated carbon, silica, alumina,etc. The search for new sorbent materials forms a continuing focus ofthe gas products industry, particularly for the production of commodityindustrial gases such as oxygen, nitrogen, argon, etc.

By way of specific example, systems for the commercial production ofoxygen from air by PSA or vacuum-pressure swing adsorption (VPSA)frequently use zeolites as an adsorbent. Nitrogen is more stronglyadsorbed than oxygen on zeolites, so when high pressure air is placed incontact with these materials, an oxygen-rich atmosphere is left.Lowering the pressure over the adsorbent bed allows the adsorbednitrogen to reenter the gas phase (such desorbate then may be used as anitrogen source), and the cycle is repeated. Using vacuum in the cycle(PVSA) results in slightly better performance.

PSA and VPSA techniques alone typically deliver oxygen with a purity of90-95%, with nitrogen and argon as the major impurities. Where thispurity level is acceptable, oxygen can be generated on-site.

Oxygen, however, frequently is desired to be produced at a purity levelon the order of 99+%, and this is difficult to achieve economically incommercially available PSA and VPSA systems.

Polymeric membrane processes have been suggested as a potential solutionto this problem, in view of the conceptually low capital costs, smallsize, light weight and simple operation of membrane-based separationsystems. Nonetheless, efforts to produce oxygen economically withpolymeric membranes have not been successful, as a result of poorpermeation selectivity in commercially available polymeric membranes. Inconsequence, current polymeric membrane systems are not available toproduce oxygen in high purity. Single-pass membrane units deliver 35-40%oxygen. Multiple pass units can go over 90%, but are not able toeconomically reach the aforementioned high purity threshold of 99+%.

SUMMARY OF THE INVENTION

The present invention provides a ceramic composition having highadsorptive affinity for oxygen at elevated temperature. Such ceramiccomposition may be usefully employed as an adsorbent medium in a PSAsystem to economically and efficiently sorptively remove oxygen from anoxygen-containing feed gas mixture, and produce extremely high purityproduct gas due to the selectivity of the ceramic composition foroxygen.

In one aspect, the invention relates to a ceramic composition having ahigh adsorptive capacity for oxygen at elevated temperature, comprisinga ceramic material selected from the following group:

Bi_(2−y)Er_(y)O_(3−d);

Bi_(2−y)Y_(y)O_(3−d);

La_(1−y)Ba_(y)Co_(1−x)Ni_(x)O_(3−d);

La_(1−y)Sr_(y)Co_(1−x)Ni_(x)O_(3−d);

La_(1−y)Ca_(y)Co_(1−x)Ni_(x)O_(3−d);

La_(1−y)Ba_(y)Co_(1−x)Fe_(x)O_(3−d);

La_(1−y)Sr_(y)Co_(1−x)Fe_(x)O_(3−d); and

La_(1−y)Ca_(y)Co_(1−x)Fe_(x)O_(3−d);

wherein

x is from 0.2 to 0.8,

y is from 0 to 1.0 and

d=0.1 to 0.9.

Such ceramic composition may be in a divided form, e.g., beads, spheres,rings, toroidal shapes, irregular shapes, rods, cylinders, flakes,films, cubes, polygonal geometric shapes, sheets, fibers, coils,helices, meshes, sintered porous masses, granules, pellets, tablets,powders, particulates, extrudates, cloth or web form materials,honeycomb matrix monolith, composites with other components, orcomminuted or crushed forms of the foregoing conformations.

The ceramic composition may additionally be coated on a substrate, suchas a support in one of the conformations mentioned in the precedingparagraph.

The ceramic composition of the invention may be utilized in a dividedform in a vessel, to provide a sorptive unit that may be employed in aprocess system for separation of oxygen from an oxygen-containing feedgas mixture that is flowed through the vessel for contacting with theceramic composition.

In a further aspect, the invention relates to a ceramic compositionhaving a high adsorptive capacity for oxygen at elevated temperature, inwhich the composition comprises a material selected from the groupconsisting of:

Bi_(1.55)Er_(0.45)O_(3−d);

Bi_(1.5)Y_(0.5)O_(3−d);

La_(0.6)Sr_(0.4)Co_(0.8)Ni_(0.2)O_(3−d);

La_(0.6)Sr_(0.4)Co_(0.6)Ni_(0.4)O_(3−d);

La_(0.6)Sr_(0.4)Co_(0.4)Ni_(0.6)O_(3−d);

La_(0.6)Ba_(0.4)Co_(0.8)Fe_(0.2)O_(3−d);

La_(0.6)Sr_(0.4)Co_(0.8)Fe_(0.2)O_(3−d); and

La_(0.6)Ca_(0.4)Co_(0.8)Fe_(0.2)O_(3−d);

wherein d=0 to 0.5.

In another aspect, the invention relates to a method of making a ceramiccomposition having a high adsorptive capacity for oxygen at elevatedtemperature, in which the composition comprises a material selected fromthe group consisting of:

Bi_(2−y)Er_(y)O_(3−d);

Bi_(2−y)Y_(y)O_(3−d);

La_(1−y)Ba_(y)Co_(1−x)Ni_(x)O_(3−d);

La_(1−y)Sr_(y)Co_(1−x)Ni_(x)O_(3−d);

La_(1−y)Ca_(y)Co_(1−x)Ni_(x)O_(3−d);

La_(1−y)Ba_(y)Co_(1−x)Fe_(x)O_(3−d);

La_(1−y)Sr_(y)Co_(1−x)Fe_(x)O_(3−d); and

La_(1−y)Ca_(y)Co_(1−x)Fe_(x)O_(3−d);

where

x is from 0.2 to 0.8,

y is from 0 to 1.0 and

d=0 to 0.9,

wherein the method comprises forming a mixture of respective oxalates ofthe corresponding metals in the ceramic composition, and calcining themixture to form the ceramic composition as a crytalline oxide.

The mixture of respective oxalates of the corresponding metals in theceramic composition is advantageously formed by combining thecorresponding metals with oxalate ion, with the oxalate ion being formedby heating glycolic acid. The glycolic acid in turn may be produced byreaction of nitric acid and ethylene glycol.

In yet another aspect, the invention relates to a method of making alanthanum calcium cobalt metal oxide, LaCaCoMO, wherein the metal M isselected from the group consisting of iron and nickel, comprising thesteps of:

(a) reacting nitric acid and ethylene glycol to yield glycolic acid;

(b) heating the glycolic acid to form oxalate ion;

(c) reacting the oxalate ion with the metals La, Ca, Co and M to form amixture of oxalates of said metals; and

(d) calcining the mixture of oxalates of the aforementioned metals toyield the lanthanum calcium cobalt metal oxide, LaCaCoMO.

As used herein, the term “high adsorptive capacity” means an oxygenstorage of at least 40 millimoles of oxygen per mole of the ceramicmaterial when the ceramic material is contacted with oxygen gas at atemperature of 800° C.

As used herein, the term “elevated temperature” means a temperature inthe range of from 400° C. to 1000° C.

When metal oxide ceramics are referred to herein in symbolic notationalform without stoichiometric subscripts (e.g., in the term LaCaCoMO), itis to be understood that the respective elemental constituents arepresent in such material in stoichiometrically appropriate proportionsrelative to one another.

Additional objects, features and advantages of the invention will bemore fully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a ceramic particle showing anassociated concentration profile of oxygen as a function of radius ofthe particle.

FIG. 2 is a graph of oxygen storage capacity as a function of time ofexposure to the oxygen-containing feed gas mixture, for particles ofceramic adsorbent having different diameters.

FIG. 3 is a graph of weight gains of the LaCaCoNiO perovskite oxide asdetermined by thermogravimetric analysis (TGA) and furnace oxidation at800° C.

FIG. 4 is a schematic representation of a pressure swing adsorptionsystem using a ceramic adsorbent according to one embodiment of thepresent invention.

FIG. 5 is a graph of moles of oxygen retained in or removed from acrystal lattice of a ceramic adsorbent of a type advantageously employedin the use of the ceramic material of the invention, as a function ofpartial pressure of oxygen (Po₂ (bar)) or electrical potential imposedon the ceramic adsorbent (E(V)).

FIG. 6 is a perspective view of a coated fiber article including a fibersubstrate coated with a ceramic adsorbent material according to oneembodiment of the invention.

FIG. 7 is a simplified schematic representation of a multibed PSA systemutilizing a ceramic adsorbent according to one embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

Ceramic materials have been long known for their ability to dissociateoxygen molecules and transport ionic oxygen through their crystallattice. All known efforts to exploit this phenomenon have focused onusing the ceramic material as a separation membrane. However, thedevelopment of such membranes to a commercial scale is fraught withdifficulties because of the inherent problems of using ceramics asstructural members. The brittle nature of ceramics, e.g., their poorductility, impact toughness and tensile/compressive characteristics,severely limit their use where tensile forces and thermal shock areencountered. As a membrane is subjected to high pressures andtemperatures, typical failure modes of the membranes include thermalstress cracks, catastrophic failures, pinhole leaks, and failures thatresult from runaway thermal reactions on the surface of the membrane.

The present invention relates to ceramic compositions that are useful asa selective adsorption medium. Such ceramic compositions may be utilizedas finely divided materials or as coatings on substrate elements toprovide an adsorbent mass. The adsorbent mass may be deployed in avessel through which an oxygen-containing feed gas mixture is flowed toremove oxygen from the gas mixture, e.g., as part of a pressure swingadsorption (PSA) system. Such usage of the ceramic material obviates thedeficiencies described hereinabove for ceramic membranes.

In conventional ceramic membrane systems a ceramic material is used totransport oxygen ionically, using a ceramic material having a capacityfor oxygen. But instead of using the ceramic material as a membranephysically separating oxygen from air, the ceramic material of thepresent invention may be used to temporarily store oxygen to effectseparation during a pressure swing adsorption process. By eliminatingthe function of a “separating membrane,” the development problemsassociated with scale-up, hot spots, thermal stresses, and manufacturingyields all disappear.

More specifically, the ceramic compositions of the present invention maybe utilized as a ceramic ionic transport material in a pressure swingadsorption system to trap and deliver oxygen. Rather than useelectricity to drive the transport of oxygen, as in conventional ceramicmembrane separation processes, the ionic transport ceramic alone can actas an ionic oxygen storage medium.

In this manner, the ceramic compositions of the invention are ionictransport materials that may be employed as efficient “absorbents” withsole selectivity for oxygen. For example, by heating ceramic sorbentparticles of the instant composition and maintaining same at elevatedtemperature, oxygen will be removed from an air stream flowed through abed of such particles. Then, by reducing the pressure, theoxygen-deficient air stream can be removed, and oxygen can subsequentlybe liberated from the ceramic to yield a pure oxygen source. By thisabsorbent arrangement, oxygen is selectively and efficiently removedfrom the gas stream.

The ceramic compositions of the present invention enable the bulkseparation and purification of oxygen based on ionic transport, in whichthe ceramic adsorbent is maintained at high temperature to temporarilystore oxygen. Oxygen that contacts the surface of the ceramic adsorbentdecomposes on the surface and is incorporated into the crystallinelattice of the ceramic material. While such process requires hightemperatures, e.g., on the order of 600-900° C., the higher separationefficiency of oxygen from other (inert) gases in the oxygen-containingfeed gas mixture is unexpectedly superior to conventional PSAapproaches.

When the oxygen gas contacts the ceramic adsorbent, there is adsorptionand dissociation of the oxygen, with charge transfer acting to causepenetrative flux of the oxygen species into the sorbent materialparticle. The chemical potential driving force is therefore employed toeffect ionic transport of the oxygen species into the sorbent material.

The ceramic oxide adsorbent particles may be constituted by an inertsubstrate that is coated or otherwise associated with the ceramicmaterial.

The ceramic adsorbent material of the present invention may be of anysuitable size, shape and conformation appropriate to the end useapplication and the specific feed gas mixture involved in the oxygenadsorption use of the material. For example, the material may be in afinely divided form, e.g., beads, spheres, rings, toroidal shapes,irregular shapes, rods, cylinders, flakes, films, cubes, polygonalgeometric shapes, sheets, fibers, coils, helices, meshes, sinteredporous masses, granules, pellets, tablets, powders, particulates,extrudates, cloth or web form materials, honeycomb matrix monolith,composites (of the ceramic adsorbent with other components), orcomminuted or crushed forms of the foregoing conformations.

The ceramic adsorbent materials of the invention may be formed bymetalorganic chemical vapor deposition (MOCVD) on suitable supports orsubstrates, using appropriate precursors for the respective metalcomponents of the metal oxide ceramic materials. Usage of MOCVD permitsclose control of stoichiometry and uniformity of coverage to beachieved. MOCVD permits films of multicomponent ceramics to be depositedwith compositional reproducibility on the order of 0.1% and thicknessuniformity of better than 5%.

Alternatively, the ceramic adsorbent material may be formed as bulkarticles, e.g., particles, by conventional ceramic manufacturingtechniques, such as powder metallurgy, slurry metallurgy (slip casting,tape casting, etc.) and coextrusion.

Another technique for forming the ceramic adsorbent utilizes sol geltechniques, which are particularly advantageous when the ceramicmaterial is deposited on a carrier or inert substrate, such as a poroussilica, alumina, kieselguhr, or the like. Sol gel techniques may beemployed to make up a sol of the ceramic precursor and to spray,dip-coat, soak, roller coat, or otherwise apply the solution to thesubstrate, following which the mixed cation gel can be subjected to hightemperature, e.g., calcined, to produce the desired ceramic material.

Sol gel techniques can also be used to form the ceramic as anunsupported article, such as in a fluidized calcining bed, or as formedon a lift-off substrate formed of or coated with a low surface energycoating to facilitate separation of the ceramic product from thelift-off substrate.

When MOCVD is employed for ceramics formation, the conventional approachfor compositional control over a complex oxides is to introducemetal-organic precursors to the reactor via independently controlledmanifolds, each requiring accurate control of temperature, pressure,flow rates and precursor concentrations. Besides its complexity, thismethod makes film stoichiometry highly sensitive to inaccuracies in anyof these process variables. Films that are rich in specific metals willcontain local non-stoichiometric phases or amorphous oxides thatcontribute to film inhomogeneity.

The use of MOCVD is successful in overcoming these composition controlproblems, as well as the difficulties encountered when moderatevolatility precursors—often solids—need to be used. In the latterinstance, the MOCVD process may employ either neat liquids or liquidsolutions (if solid precursors are used) in a single mixture containingall the components desired in the film. Such “liquid delivery” thereforeis an advantageous method of forming the ceramic adsorbent elements.

In forming films of the ceramic adsorbent material on substrate elementsin the practice of the invention, individual precursors required forphase stability can be intimately mixed in a ratio that produces thedesired film composition based on their respective deposition rates.Using liquid delivery the liquid sources are flash vaporized, therebyspending little time at the high temperature that leads todecomposition. Precursor compositions are commercially available for themetal species of the ceramic adsorbent compositions of the invention,e.g., Zr, Y, Sr, Bi, Fe, La, Co, Ca precursors, which are provided inliquid form as precursors suitable for liquid delivery MOCVD. Liquiddelivery precursors of such type are commercially available fromAdvanced Technology Materials, Inc. (Danbury, Conn.) under the trademarkEPIGRADE. Examples of such types of precursors include those disclosedin U.S. Pat. Nos. 5,453,494, 5,840,897, and 5,280,664, and U.S. patentapplication Ser. No. 08/484,654, now U.S. Pat. No. 6,110,529, thedisclosures of which hereby are incorporated herein in their respectiveentireties.

Using the ceramic materials of the invention as absorbents in fixed bedPSA systems offers several advantages that currently hamper thedevelopment of such materials as membranes:

Reliability—Hot spots can develop in ceramic membranes as a result ofexothermic recombination reactions on the surface of a nonuniformmembrane. The hot spots then exhibit higher oxygen diffusion ratesleading to runaway reactions and film failures. This problem can existin newly manufactured membranes, and can develop in membranes exposed tomany hours of use. By utilizing the ceramic adsorbent material in a PSAarrangement, pin holes, film uniformity, thermal expansion mismatches,and sealing problems no longer are relevant.

Easy Scale Up and Development—The active ceramic material in the presentinvention is not employed as a structural operational element where itsphysical integrity is important for function. Structural applicationsfor ceramics are notorious for scale-up problems. With a fixed bed,scale-up is straightforward and economical. The components of the PSAequipment are well known and their reliability is proven.

Large Surfaces Areas—By properly choosing the particle size andporosity, the surface area of the ceramic adsorbent bed can be adjustedto maximize mass transport for specified pressure swings and diffusiondistances.

High Purity Inert and Oxygen Streams—Because the ceramic adsorbentmaterial has perfect selectivity for oxygen, both high purity oxygen andinert streams can be produced. Conventional air separation PSA utilizesthe differences in quadrapole moments between nitrogen and oxygen forseparation, and since argon does not have a quadrapole moment it is notseparated from oxygen. As a ceramic sorbent will only absorb oxygen, theinert components such as nitrogen and argon are left behind in the gasprocessed by contacting with the ceramic sorbent, and do not contaminatethe product gas.

Rather than use an electric potential to drive ceramic materialmembranes, the ceramic materials of the present invention may bedeployed in a form that utilizes a concentration gradient to storeoxygen. By having a pressure or concentration gradient over thematerial, a concentration profile is developed within the film. Whetherthe ceramic is a film or particle, in the presence of air at hightemperature, oxygen will diffuse into the structure and create acompositional gradient as shown schematically in FIG. 1. Then byreducing the pressure or by using a vacuum, oxygen will diffuse fromsurface, thus providing a supply of pure oxygen.

A PSA process using the ceramic compositions of the present inventionmay be carried out at any suitable pressure levels in the respectiveadsorption (“loading” of the ceramic adsorbent) and desorption(releasing of oxygen from the ceramic adsorbent), as may be readily bedetermined within the skill of the art and without undueexperimentation. The adsorption pressure may by way of illustrationentail a pressure level on the order of from about 1.2 to about 10atmospheres, and the corresponding desorption pressure may be in therange of from about −0.2 atmosphere to about 0.8 atmosphere, at atemperature that is appropriate to the process, feed gas mixture andoxygen product characteristics.

The temperature maintained in the PSA process in one preferred aspect ofthe invention may be on the order of from about 400° C. to about 900° C.The adsorbent beds may be maintained in a heated condition by anysuitable heating or thermal energy input means, such as for exampleelectrical resistance heating elements disposed in the adsorbent bed,jacketing of the adsorbent vessels of the PSA system with a suitableheat exchange fluid being flowed through the jacket to maintain thevessel and adsorbent bed contained therein at a selected temperature.Other means include elements, assemblies and subassemblies for effectingradiant heating, ultrasonic heating, microwave heating, convectiveheating, conductive heating (e.g., via extended surface elements such asfins in the interior volume of the adsorbent bed, coupled in heatedrelationship with a thermal energy source), heat exchangers, etc.

The adsorbent beds in the PSA process system may be suitably sized forany appropriate output of oxygen for the given oxygen-containing feedgas mixture that is processed in the system, and the appertaining cycletime. For example, the system may be sized to produce 120 standard cubicfeet per minute (SCFM) of oxygen at 99.95% purity from anoxygen-nitrogen mixture, during a 2 minute PSA cycle.

The vessel containing the ceramic adsorbent bed may be constructed ofany suitable material, such as a refractory metal. Alternatively, thevessel may be lined with an insulator material and heating elements maybe incorporated on the inside of the vessel, to maintain the temperatureof the bed at suitable elevated temperature.

The PSA process utilizing the ceramic compositions of the presentinvention differs from the usual PSA system in that the media hasessentially perfect selectivity for oxygen, with no interaction withnitrogen or argon. In conventional PSA air separation systems usingaluminosilicate zeolites, the zeolites show thermodynamic selectivityfor nitrogen over oxygen. But since there is little difference betweenoxygen and argon, separation of these two gases is impractical withzeolites as an absorbent. Accordingly, oxygen-argon mixtures can bereadily separated using the ceramic adsorbent materials of the presentinvention in a PSA system.

More broadly, such PSA systems may be advantageously employed to produceproduct gases such as oxygen, deoxygenated gases or gas mixtures (wherethe feed gas mixture undesirably contains oxygen), nitrogen (as thenon-adsorbed gas from an oxygen-nitrogen mixture), etc. For example, aPSA system of such type may be employed to form “deoxo” gas, in which a98% nitrogen stream containing oxygen is treated by the PSA process toyield a nitrogen gas product containing less than 100 parts per millionby volume of oxygen.

Using ceramic materials as separation media requires high temperature,e.g., temperatures on the order of 800° C.

The ceramic adsorbent material may be coated in a thin film by MOCVD onan inert substrate, e.g., a particulate substrate, fibrous substrate,sheet or other substrate formed of a material such as active alumina,gamma alumina, tabular alumina or fused alumina. Alternatively, thecoating of the substrate may be carried out by spray, sol gel,soak-and-bake, solution deposition, dipping, roller coating, or anyother suitable technique. Such coated substrate bodies can be employedfor pressure swing separation of oxygen-containing feed gas mixtureswhen such bodies are aggregated in a bed or mass with which the feed gasmixture is contacted at higher pressure, for subsequent release of theoxygen gas at lower pressure.

Various ionic transport electrolyte materials are identified in Table 1below wherein surface exchange rates, diffusion coefficients, and mostimportantly, maximum storage capacities are set forth. As shown in thetable, La_(0.6)Ca_(0.4)Co_(0.8)Fe_(0.2)O_(3−δ), with the highest surfaceexchange rate, has a theoretical storage capacity of 150 mmol O₂/mol at800° C.

FIG. 2 shows the theoretical oxygen storage capacity and rate responseof this material for different sized particles. A storage capacity ofabout 100 mmol O₂/mol (1.6% weight gain) is shown for this material at areaction time of around 10 minutes.

TABLE 1 Experimental reported values for K and D, and some modelingresults Surface Diffusion Max. Temp. Ex. Rate Coeff., Storage Vacancy %Composition (° C.) K (m/s) D (m²/s) (mmol O₂/mol) Filled in 100sZr_(0.84)Y_(0.16)O_(2-d) — YSZ 700 6 × 10⁻¹¹ 2.5 × 10⁻¹³    40 0.13 YSZ— Bi Implanted 700 1.6 × 10⁻¹⁰   1.4 × 10⁻¹²    40 0.15 YSZ — FeImplanted 700 3.4 × 10⁻¹⁰   3.3 × 10⁻¹³    40 0.66Bi_(1.55)Er_(0.45)O_(3-d) 700 1 × 10⁻⁷  4 × 10⁻⁹  250 1.76Bi_(1.5)Y_(0.5)O_(3-d) 700 5 × 10⁻⁹  9 × 10⁻¹² 250 1.85La_(0.6)Sr_(0.4)Co_(0.8)Ni_(0.2)O_(3-d) 800 2 × 10⁻⁸  1 × 10⁻¹¹ 150 6.75La_(0.6)Sr_(0.4)Co_(0.6)Ni_(0.4)O_(3-d) 800 3 × 10⁻⁸  6 × 10⁻¹² 200 12.4La_(0.6)Sr_(0.4)Co_(0.4)Ni_(0.6)O_(3-d) 800 2 × 10⁻⁸  7 × 10⁻¹² 250 7.97La_(0.6)Ca_(0.4)Co_(0.8)Fe_(0.2)O_(3-d) 700 4 × 10⁻⁸  2 × 10⁻¹² 15025.27 La_(0.6)Ca_(0.4)Co_(0.8)Fe_(0.2)O_(3-d) 800 2 × 10⁻⁷  1 × 10⁻¹¹150 44.6 La_(0.6)Ca_(0.4)Co_(0.8)Fe_(0.2)O_(3-d) 900 4 × 10⁻⁷  3 × 10⁻¹¹150 48.5

Increasing the oxygen vacancy in LaCaCoFeO compounds, throughcompositional modifications, was unsuccessful in improving their oxygenabsorption capacity. Replacement of the Fe element with Ni was found,surprisingly, to enable the manipulation of composition and storagecapacity.

FIG. 3 shows the weight gains detected, through STA and furnaceoxidation, on La_(1−x)Ca_(x)Co_(0.8)Ni_(0.2)O_(3−δ) compounds at 800° C.A 5.2% weight gain was achieved by the CaCo_(0.8)Ni_(0.2)O⁰⁻⁶⁷ compound.

Synthesis of LaCaCoNiO perovskite oxide materials may be carried out bya modified “Pechini” method, a liquid mixed technique, with ethyleneglycol and nitrates used for the synthesis of LaCaCoNi oxide powders. Atypical Pechini process involves the ability of certain weak acids(alphahydroxycarboxylic acid) to form polybasic acid chelates withvarious cations. These chelates can undergo polyesterification whenheated in a polyhydroxyl alcohol to form a polymeric glass which has thecations uniformly distributed throughout the material.

Various cationic sources, such as carbonates, hydroxides, and alkoxidescan also be used for the synthesis. Ethylene glycol reactions aresimilar to those of primary alcohols except for the presence of ahydroxyl group on each carbon. When cold nitric acid is added to theethylene glycol, it oxidizes one of the alcohol groups giving glycolicacid. Heating the mixture yields oxalic acid, which is the simplestdibasic acid, comprising just two connected functional acidic carboxylicgroups. Each carboxylic group loses a proton and forms oxalate ion. Theoxalate ion C₂O₄ ⁻² functions as a bidentate chelate with a metal atomand forms a five-member chelate ring as shown below.

The majority of the elements in the Periodic Table form oxalatecomplexes. Because of the coordinating properties of the bidentateoxalate ion, most of the metals form complex oxalates in addition tosimple oxalates. After the formation of these mixed cation gels, theyare suitably calcined at elevated temperature, e.g., at 1080° C., toform crystalline oxide.

A schematic representation of a vacuum pressure swing system 10according to one embodiment of the invention is shown in FIG. 4.

The system 10 includes a sorbent vessel 12 enclosing an interior volumecontaining a bed 14 of ceramic adsorbent material, in the form of finelydivided solids particles of such material, or other material form, suchas a bed of fibers coated with a ceramic adsorbent material. The vessel12 is reposed in a furnace 16 which is operated to maintain apredetermined temperature environment in the firnace enclosurecontaining the vessel 12, so that the bed 14 of adsorbent material ismaintained at temperature suitable for sorption of oxygen from anoxygen-containing gas mixture contacted with the bed, and for desorptionof oxygen from the ceramic material at the low pressure (depressurized)condition of the PSA process when oxygen is released from the adsorbentand discharged from the PSA bed.

At its upper end, the vessel 12 is joined to a feed and dispensingconduit 18. The conduit 18 in turn is joined by three-way valve 32 tothe main flow conduit 20.

The main flow conduit 20 has disposed therein a pump 24, which may beselectively actuated to flow gas from the bed 14 through the main flowconduit 20, either to the oxygen vessel 26 joined to the main flowconduit 20 by branch line 22 and associated three-way valve 34, or tothe argon-nitrogen vessel 30 joined to the main flow conduit 20 bytwo-way valve 36.

In operation, air or other oxygen-containing feed gas mixture from asuitable source (not shown in FIG. 1) is flowed through the main flowconduit 20 with the valve 32 directing the flow through feed anddispensing conduit 18 into vessel 12. During such charging the furnaceis actuated and maintains the ceramic adsorbent in a “hot” stateappropriate to penetration of the oxygen into the sorbent particles ofbed 14. When the bed has equilibrated, the interstitial void gas in thebed 14, comprising nitrogen and argon, and depleted in oxygen, is thenpumped by pump 24 to the argon-nitrogen vessel 30, with valve 32 beingswitched to interconnect feed and dispensing conduit 18 with thedownstream portion of main flow conduit 20, valve 34 being closed toflow into the branch line 22, and valve 36 being open to flow of gasfrom the main flow conduit 20 into branch line 28 and argon-nitrogenvessel 30.

The furnace 16 remains actuated, as valve 36 is closed to isolate theargon-nitrogen vessel 30, and valve 34 is opened to permit flow from themain flow conduit 20 to branch line 22. Pump 24 then acts to extract theoxygen gas from the vessel 12, as the ceramic adsorbent is maintained ata temperature allowing transport of the “trapped” oxygen from thesorbent bed particles into the voids of the bed. The oxygen gas then isflowed from vessel 12 through feed and dispensing conduit 18, main flowconduit 20, branch line 20 and into oxygen vessel 26.

After the adsorbent bed in vessel 12 has been extracted of the oxygen,valve 32 is switched and the feed gas mixture is again charged to thesorbent bed in vessel 12 as the furnace is maintained in operation tokeep the adsorbent at suitable elevated temperature. Concurrently, valve34 is switched to isolate oxygen vessel 26 and valve 36 is switched toopen argon-nitrogen vessel 30 to flow of non-adsorbed gas thereinto, asthe cycle is repeated.

In this manner, the system schematically shown in FIG. 4 may besequentially, repetitively and cyclically operated to effect separationof air or other oxygen-containing feed gas mixture into oxygen-depletedand oxygen fractions for recovery thereof as described.

It will be apparent that the system shown schematically in FIG. 4 isillustrative only, and that the invention may be practice in other,multiple-bed arrangements, for continuous, semi-continuous, or batchoperation, to separate the feed gas mixture involved. The heat flux maybe maintained constant throughout the entire operation of the PSAprocess, or the temperature may be modulated during the process, as maybe necessary or desirable in a given end use application of the presentinvention. For this purpose, the process system may employ suitablethermostatic, heat exchange or other temperature-controlling elements,such as thermosensors, temperature controllers, microprocessors, massflow controllers, etc.

The PSA system uses ceramic compositions according to the invention thathave the capacity to transport ionic oxygen. Molecular oxygendissociates on the surface of the oxide ceramic and is then incorporatedinto the crystalline lattice. Potential gradients, for example inconcentration, can cause the oxygen to move into and through thelattice. The imbalance in ions results in an electrical potentialgradient across the material. In this manner, concentration andelectrical gradients can be viewed as equivalent in terms of a drivingforce for oxygen transport. FIG. 5 shows this equivalence in terms ofthe moles of oxygen retained or removed from the lattice. Thus, pressure(concentration) is used to drive ionic oxygen into the ceramic adsorbentparticles.

As an alternative to PSA processing of the oxygen-containing feed gasmixture, or in addition thereto, the ceramic adsorbent material could bedeployed in a suitable form, e.g., corresponding particles in a bed ofsuch material, and a voltage could be cyclically impressed on the bed,as modulated by suitable cycle timer means, to effect sorption anddesorption of oxygen. The PSA process may therefore be conducted in avoltage-assisted manner, to achieve a desired efficiency of theseparation process.

For example, the bed of ceramic particles may be disposed in acontainment vessel containing a spaced-apart array of screen or gridelectrode elements, containing ceramic adsorbent particles between thesuccessive elements, and with the electrode elements alternatinglycoupled to a voltage supply and ground, to provide a circuitryarrangement which can adsorb and then desorb the oxygen from anoxygen-containing gas.

FIG. 5 is a graph of moles of oxygen retained in or removed from acrystal lattice of a ceramic adsorbent, as a function of partialpressure of oxygen (Po₂ (bar)) or electrical potential imposed on theceramic adsorbent (E(V)). The ceramic adsorbent material is a metalborate material of the formula La_(1−y)A_(y)BO_(3+x), wherein A may be atransition metal, y is from 0 to 1, and x is from 0 to 1. Theperformance of such metal borate ceramic is illustrative, and higheroxygen working capacities are achieveable by the ceramic compositions ofthe present invention.

The ceramic composition of the present invention is a ceramic oxidematerial through which only oxygen can diffuse. The composition of theceramic oxide adsorbent material is such that a significant number ofoxygen vacancies exist in the material. These oxygen vacanciesfacilitate the selective diffusion of oxygen through the material atrelatively high rates. By placing either a voltage potential or apressure gradient across the membrane, oxygen is selectively diffused inand through the oxide material. The oxide composition has the followingfeatures: 1) a high concentration of oxygen vacancies, 2) thermodynamicstability enabling operation at temperatures over 600° C., and 3)thermodynamic stability enabling operation under highly reducingconditions.

FIG. 6 is a perspective view of a coated fiber article 50 including afiber substrate 52 coated with a ceramic adsorbent material 54 accordingto one embodiment of the invention. Coated fibers of such type may beemployed in a bed of such fibers in a PSA process for extraction ofoxygen from an oxygen-containing feed gas mixture. Alternatively, suchfibers may be formed into woven or non-woven fibrous webs, that maylikewise be employed in a PSA process for take-up of oxygen from anoxygen-containing feed gas mixture.

The fiber substrate 52 may be formed of a porous alumina material. Thecoating 54 may be an LCCFO material (LaCaCoFeO) deposited on the fibersubstrate at a thickness on the order of 0.1 micrometer. Such coatedfiber article may be readily formed by liquid delivery MOCVD techniquesusing suitable precursors for the lanthanum, calcium, cobalt, and ironcomponents of the LCCFO film. An alternative coating material maycomprise an LCCNO material (LaCaCoNiO).

Alternatively, the coated fiber article may be formed by sol geltechniques, or in any other suitable manner.

The PSA system utilizing ceramic composition of the invention embodies ahighly efficient means and method for the extraction of oxygen from anoxygen-containing feed gas mixture, by contacting such feed gas mixturewith the ceramic sorbent material having sorptive affinity for oxygen atelevated temperatures, such as in the range of 400° C.-1000° C. The PSAsystem may be configured in any suitable manner, with any appropriatenumber and size of beds for separation of oxygen from anoxygen-containing feed gas mixture. The PSA system may be controlled bysuitable cycle timer units, e.g., integrated with computer control, toprovide appropriate continuous, semi-continuous or batch operation.

FIG. 7 is a simplified schematic representation of a multibed PSA system100 comprising two adsorbent vessels, A and B, each of which is filledwith the ceramic adsorbent material, and employed for the production ofnitrogen as a product gas, from an oxygen/nitrogen feed gas mixture.

The series of valves connecting the adsorbent vessels A and B may bedefined by the number shown in the drawing and by the function performedin the following preferred arrangement:

Valve(s) Description (a) Valves 101 and 102 inlet air valves toadsorbent vessels “A” and “B” respectively. (b) Valves 103 and 104depressurization valves (c) Valve 105 product flow valve from vessels“A” and “B” to the product tank (d) Valve 106 product gasrepressurization valve from product tank to repressurizing vessel (e)Valve 107 product gas purge valve from product tank through the vesselunder purge (f) Valves PCV-1, PCV-2 pressure reduction (back pressurecontrol) valves. (g) Check valves control as flow directions. These areshown as arrows between the vessel connecting means. Gas flows in thedirection of the arrow. (h) Restriction orifice shown above valve 107.Restricts gas flow for purge.

The oxygen-containing feed gas mixture to be separated, e.g., air, iscompressed and introduced into the system via either valve 101 or valve102.

The feed gas mixture may be modified, prior to adsorption, by passing itthrough a dryer to remove excess humidity as a significantly reducedrelative humidity may be preferred. Additionally, a filter or scrubbermay be employed to remove other gases such as carbon dioxide or oxidesof nitrogen. These steps improve the purity of the feed gas mixture.

Feed air is admitted to either vessel A or vessel B as a compressed gasvia either valve 101 or valve 102 to selectively remove oxygen as thefeed air flows cocurrently through the ceramic adsorbent. The PSAprocess will operate within a wide range of actual pressures, e.g., anadsorption pressure from the range of 3.0 to 8.0 bars in thisembodiment. The elevated temperature process condition in the adsorbentvessels is maintained by suitable heating means (not shown in FIG. 7),which maintain the temperature of the sorbent beds in the respectivevessels at a level on the order of from about 700-900° C. Nitrogenproduct gas is discharged from column A, for example, through valvePCV-1, via valve 105 and is collected in the product tank. The productnitrogen gas oxygen concentration may be analyzed upstream of theproduct tank as a measure of instantaneous product gas purity, ordownstream of the product tank as a measure of average product gaspurity. A flow of product gas is discharged from the product tank at aconstant pressure somewhat lower than the minimum pressure of theproduct tank. This is accomplished via pressure reducing valve PCV-2.

Each adsorbent vessel is cycled through adsorption, partialequalization, depressurization, purge, product repressurization and feedrepressurization steps. One system cycle is defined as the completion ofthese steps for both columns.

At the conclusion of each vessel's adsorption cycle, the nearly spentadsorbent vessel is partially vented at its inlet (or bottom) and thevented gas is passed to the bottom (or inlet) of the column to berepressurized. This partial venting occurs substantially simultaneouslywith the cocurrent feed gas repressurization of the column beingregenerated for adsorption by opening valves 101 and 102.

Following this partial “bottoms” equalization step, the nearly spentadsorbent vessel, B, is isolated and is totally depressurized toatmospheric pressure at its inlet via valve 104 thereby desorbing andexhausting quantities of byproduct exhaust, i.e., adsorbed oxygen. Thevented column is then countercurrently swept with 0.1 to 1.0 bed volumeof product gas at a controlled flow from the product tank introduced viavalve 107 to purge the adsorbent vessel of additional residual andadsorbed oxygen via valve 104. The isolated adsorbent vessel B is thenpartially regenerated by repressurizing with product gas from theproduct tank via valve 6 to from 10% to 30% of the adsorption pressure.

Final repressurization of the regenerated adsorbent vessel isaccomplished by the substantially simultaneous introduction of ventedgas from the bottom of the column, which has completed its adsorptioncycle, and compressed feed air, via open valves 101 and 102 until from40% to 80% of the adsorption pressure is reached, after which valve 101is closed.

The system cycle is continuously repeated alternatively using one columnfor the production of enriched nitrogen while the second column isregenerated and repressurized.

The features and advantages of the invention are more fully shown withreference to the following non-limiting example, wherein all parts andpercentages are by weight, unless otherwise expressly stated.

EXAMPLE I

A PSA system including an adsorbent bed containing LCCFO ceramicmaterial is arranged with an upstream heat exchanger and is employed toproduce nitrogen at high purity.

Influent air is passed through the heat exchanger in countercurrent flowwith nitrogen-rich product gas discharged from the adsorbent bed. Theresultantly heated influent air stream flows from the heat exchanger tothe adsorbent bed and oxygen is removed therefrom by the adsorbent inthe bed, with the bed being heated by suitable means. In thedepressurized state after active removal of oxygen from the aircontacted with the adsorbent, the oxygen is pumped from the bed by apump arranged downstream from the adsorbent bed and discharged from thesystem for packaging, transport and/or end use.

The influent air stream has a mole fraction composition of oxygen, 20.9mol. %, nitrogen, 78 mol. %, argon, 1.0 mol. % and carbon dioxide, 0.10mol. %. This stream is at a temperature of 25° C. and a pressure of 50psia. The temperature of the adsorbent bed is at 800° C.(alternatively,a value in the range of from about 600° C. to about 900° C. may beusefully employed). The feed air stream is heated in the heat exchangerupstream of the adsorbent bed to a temperature of 483° C. and is at apressure of 100 psia.

The nitrogen-rich gas yielded by the adsorption of oxygen from the feedair mixture and discharged from the adsorbent bed is at a temperature of800° C. and a pressure of 130 psia. After passage of the nitrogen-richgas through the countercurrent heat exchanger, the nitrogen-rich gas isdischarged from the process system at a pressure of 50 psia and atemperature of 200° C.

The oxygen-rich gas released from the adsorbent bed during the lowpressure step of the PSA process is at a pressure of 0.5 psia and atemperature of 800° C. The final oxygen-rich gas discharged from thedownstream pump is at a pressure of 1 psia.

The oxygen-rich gas contains 96.32 mol. % oxygen, 3.63 mol. % nitrogen,0.05 mol % argon, and no carbon dioxide.

The product nitrogen-rich gas contains 98.35 mol. % nitrogen, 0.27 mol.% oxygen, 1.26 mol. % argon, 0.13 mol. % carbon dioxide. The productnitrogen-rich gas may be employed for inerting applications, blanketgas, sparging gas, or other commercial inert gas applications.

EXAMPLE II

A ceramic adsorbent sol gel was employed to coat an alumina fiber matusing soak-and-bake or spray techniques. Usage of the solid fiber matsubstrates offers several advantages, including reduction of pressuredrop and maximum usage of the ceramic adsorbent medium. By coating theceramic adsorbent medium on porous alumina substrates, an oxygendiffusion distance of 80 micrometers was achieved at 800° C.

While the invention has been illustratively described herein withreference to various embodiments and disclosed features, it will beappreciated that the invention is not thus limited, but rather extendsto and encompasses numerous variations, modifications and otherembodiments. Accordingly, the invention is intended to be broadlyconstrued and interpreted as including all such variations,modifications and other embodiments within the spirit and scope thereof,as hereinafter claimed.

What is claimed is:
 1. A ceramic composition having a high adsorptivecapacity for oxygen at elevated temperature, said composition comprisingLa_(1−y)Ba_(y)Co_(1−x)Ni_(x)O_(3−d), wherein x is from 0.2 to 0.8, 0<y≦1and d is 0.1 to 0.9.
 2. A ceramic composition having a high adsorptivecapacity for oxygen at elevated temperature, said composition comprisingLa_(0.6)Sr_(0.4)Co_(0.4)Ni_(0.6)O_(3−d).
 3. A ceramic composition havinga high adsorptive capacity for oxygen at elevated temperature, saidcomposition comprising a material selected from the group consisting of:Bi_(1.55)Er_(0.45)O_(3−d); and Bi_(1.5)Y_(0.5)O_(3−d); wherein d=0.5 to0.9.
 4. A method of making a ceramic composition having a highadsorptive capacity for oxygen at elevated temperature, said compositioncomprising a material selected from the group consisting of:Bi_(2−y)Er_(y)O_(3−d); Bi_(2−y)Y_(y)O_(3−d);La_(1−y)Ba_(y)Co_(1−x)Ni_(x)O_(3−d);La_(1−y)Sr_(y)Co_(1−x)Ni_(x)O_(3−d); andLa_(1−y)Ca_(y)Co_(1−x)Ni_(x)O_(3−d); wherein x is from 0.2 to 0.8, 0<y≦1and d=0.1 to 0.9, wherein said method comprises; combining nitric acidand ethylene glycol to form a glycolic acid solution; heating saidglycolic acid solution to form oxalate ion; combining the oxalate ionwith corresponding metals to form respective oxalates of thecorresponding metals; mixing said respective oxalates of thecorresponding metals to form an oxalates mixture; and calcining themixture to form said ceramic composition as a crystalline oxide.
 5. Amethod of making a lanthanum calcium cobalt metal oxide,La_(1−y)Ca_(y)Co_(1−x)Ni_(x)O_(3−d), comprising the steps of: (e)reacting nitric acid and ethylene glycol to yield glycolic acid; (f)heating the glycolic acid to form oxalate ion; (g) reacting the oxalateion with the metals La, Ca, Co, and M to form a mixture of oxalates ofsaid metals; and (h) calcining the mixture of oxalates of said metals toyield the lanthanum calcium cobalt metal oxide,La_(1−y)Ca_(y)Co_(1−x)Ni_(x)O_(3−d) wherein x is from 0.2 to 0.8, 0<y≦1,and d=0.1 to 0.9.
 6. A ceramic composition having a high adsorptivecapacity for oxygen at elevated temperature, said composition comprisinga material selected from the group consisting of: Bi_(2−y)Er_(y)O_(3−d);and Bi_(2−y)Y_(y)O_(3−d); wherein x is from 0.2 to 0.8, y is from 0 to1.0 and d=0.5 to 0.9.
 7. The ceramic composition of claim 6, comprisingBi_(2−y)Er_(y)O_(3−d).
 8. The ceramic composition of claim 6, comprisingBi_(2−y)Y_(y)O_(3−d).
 9. A ceramic composition having high adsorptivecapacity for oxygen at elevated temperature, said composition comprisingCaCo_(0.8)Ni_(0.2)O_(2.4).
 10. A ceramic composition having a highadsorptive capacity for oxygen at elevated temperature, said compositioncomprising La_(1−y)Ca_(y)Co_(1−x)Ni_(x)O_(3−d) wherein y is from 0.6 to1.0.